Extrusion devices having multiple heads are known. In such a device, two or more extruders are disposed adjacent one another, each device having its own drive means. The outlets from the extrusion device communicate with a front template defining flow channels which converge towards one another. These flow channels lead to a common discharge orifice. Such a device is disclosed in German Offenlegungsschrift No. 3 238 284. In such a device, the extruders have differing throughput capacities. Accordingly the rubber or plastics material mixtures issuing from the individual extruders may emerge from the common nozzle at different times, particularly when the extrusion device is initially started. A considerable amount of material is thus conveyed through the head until all of the mixtures being used to form the profile being extuded, emerge from the multiple extrusion head in their full volume. The material extruded until this occures constitutes waste material which is only re-usable to a very limited extent.
An additional complication resides in the fact that it is frequently necessary to change one or more of the component mixtures during each working shift. Thus, it is not an uncommon occurrence for such a change to be effected eight to ten times. This means that a large amount of waste material is produced each time the extrusion device is restarted.
An apparatus for producing extruded profiles from different rubber or plastics material mixtures is disclosed in German Offenlegungsschrift No. 3 333 105. In such apparatus, the extruders are disposed one above the other and operate with a common extrusion head.
To prevent damage being sustained by the thin projection members forming part of the front templates disposed in the multiple head, the actuating of the drive motors of the individual extrusion devices is effected in mutual dependence upon one another and/or in a timed sequence. Such an apparatus prevents the projection members of the front templates from being bent as a result of the high pressures produced in the multiple head.
By actuating the individual drive motors successively and/or by actuating them in an inter-dependent manner, however, an extremely large amount of waste material is produced. This is because, initially, the extrusion device operating at the lower pressure is actuated first and only after a pre-selected length of time is the extrusion device operating at the higher pressure subsequently actuated.
It is also open to question as to whether such an arrangement prevents the projection members of the front templates from becoming damaged. In any event, an unacceptably high level of waste material is extruded and the need to change, for example, the principal mixture for a tread surface of the tread strip of a vehicle tyre eight to ten times during one working shift is not viable.